Meckel-Gruber Syndrome Health Article

Definition

Meckel-Gruber syndrome (MGS) is an inherited condition that causes skull abnormality, enlarged cystic kidneys, liver damage, and extra fingers and toes. Findings vary between affected infants (even in the same family), as well as between ethnic groups. Infants with MGS are usually stillborn or die shortly after birth.

Description

The first reports of MGS were published in 1822 by Johann Friedrich Meckel. G. B. Gruber also published reports of MGS patients in 1934 and gave it the name dysencephalia splanchnocystica. MGS is also known as Meckel syndrome and Gruber syndrome.

MGS affects many different organ systems including the central nervous system (brain and spinal cord), face, kidneys, liver, fingers and toes, and occasionally the bones of the arms and legs. Some researchers believe that abnormal development and differentiation of the embryonic mesoderm (the early tissue layer that contributes to the formation of the bones, cartilage, muscles, reproductive system, blood cells, heart, and kidneys) is related to MGS. The cells of the mesoderm must divide, migrate, associate, and specialize in a precise manner to form these body parts. Any problem in any step of the process can lead to multiple abnormalities in various organ systems.

Since MGS causes severe birth defects and death in the newborn period, it can be devastating for families. Extensive examination and autopsy is often needed to confirm a diagnosis of MGS, delaying the family's answers regarding their child's death. Most parents do not know they are at risk until they have a child with MGS. This can cause feelings of anger, disbelief, and guilt.

Genetic profile

The autosomal recessive inheritance pattern in MGS is well-documented. MGS affects males and females equally. Parents of affected children are assumed to be carriers and have a 25% chance of MGS recurrence in each pregnancy. A healthy brother or sister of an affected child has a two-thirds chance of being an MGS carrier.

Research involving families in Finland (where MGS is more common) led to the first MGS gene being mapped (localized) to the short arm of chromosome 17. This means that the gene location has been narrowed down to a small potential area, but the exact location and precise details about the gene are still unknown. Non-Finnish families did not show evidence of a causative gene linked to chromosome 17. This led to the search for a second MGS gene. Studies of Northern African and Middle Eastern families resulted in the second MGS gene being mapped to the short arm of chromosome 11. More research is being performed to learn more about the precise location of both MGS genes, gene changes that cause MGS, and the role of the genes in early development.

Demographics

MGS has an estimated incidence between one in 13,000 births and one in 140,000 births. This means that between one person per 50 and one person per 180 is an MGS carrier. The incidence varies among ethnic groups. Several ethnic populations have an increased incidence of MGS. The incidence in Finland is one in 9,000 births (one person in 50 is a carrier). The incidence is also higher among Belgians and Bedouins in Kuwait with one affected birth in 3,500 (one person in 30 is a carrier). The highest incidence is reported in the Gujarati Indians with one affected birth per 1,300 (one person in 18 is a carrier). The incidence among Jews in Israel is one in 50,000 (one person in 112 is a carrier). Cases of MGS have been reported in North America, Europe, Israel, Indonesia, India, Kuwait, and Japan.

Signs and symptoms

The three hallmark features of MGS are encephalocele, polycystic kidneys, and polydactyly. Approximately 90% of infants with MGS have an encephalocele. This is an opening in the skull that allows brain tissue to grow outside of the skull. Virtually 100% of infants with MGS have enlarged kidneys with cysts. Polydactyly (extra fingers and/or toes) is present in about 80% of affected children. The polydactyly is usually postaxial (the extra fingers/toes are on the same side of the hand/foot as the smallest finger/toe). In MGS, the polydactyly usually affects both the hands and feet. There may also be webbing of the fingers and toes—the skin between the fingers or toes fails to separate—leaving the digits attached to each other.

Internal examination of babies with MGS also revealed that virtually 100% have liver abnormalities. This can include halted development of the bile ducts, extra bile ducts, enlarged bile ducts, and loss of blood vessels. The liver is also usually enlarged. These liver changes are now considered by most to be another hallmark feature of MGS.

Babies with MGS often have similar facial features. Some reported features are eyes that are closer together or farther apart than usual, broad and flat nose, broad cheeks, and a wide mouth with full lips. Other features are commonly seen in MGS and are thought to be caused by a low amount of amniotic fluid surrounding the baby before birth. These features are sloping forehead, small jaw, low-set ears, and short, webbed neck. Low fluid prior to birth also frequently causes clubfoot in the newborn.

Other common features of MGS are abnormalities of the genitalia and cleft palate. The external (visible) genitalia are often small or ambiguous (not clearly male or female). There have also been reports of babies with MGS having both male and female reproductive parts (hermaphrodite). Cleft palate is seen in about 45% of babies with MGS. Cleft lip is less common but has been reported.

The symptoms of MGS are variable. Not all infants with MGS show the same signs and the characteristic signs range in severity. Some features have been described in some babies with MGS but are not as common. These include heart defects, enlarged spleen, extra spleen, hydrocephaly (extra water in the brain), absence or underdevelopment of other brain structures, and arm and leg bones that are shortened, thickened, and bowed.